The invention relates to an inductive position indicator or sensor for monitoring the relative positions of two mutually movable bodies.
Position indicators or sensors of that kind serve to produce an electrical signal, by means of which it is possible to monitor or trace and measure a continuous or intermittently occurring relative movement between two bodies, in such a way that, at any moment in time, information is available about the instantaneous position of one of the two bodies relative to the other.
Such an indicator may be a linear indicator or sensor in which for example the movement and/or instantaneous position of a machine carriage which is displaceable relative to a machine frame structure is to be detected and controlled, with a high degree of precision. That situation requires the constant production of a signal which provides information about the instantaneous position of the carriage, even when the carriage is moving at a high speed.
Another form of indicator is a rotary indicator or sensor for measuring the instantaneous angular position of a rotating body, for example the rotor of an electric motor relative to the stator, or the rotary angle between two bodies which are rotatable relative to each other, for example the azimuth or vertical angle of the telescope of a theodolite.
In a similar fashion, a rotary indicator or sensor can be used to measure the angular positions or speeds of rotation of motor vehicle wheels or the instantaneous angular position of a carburettor butterfly valve.
A linear position indicator can be found in German patent specification No. 25 11 683. That indicator has ferromagnetic flux guide means including two rectangular elongate flat plates arranged parallel to each other in such a way that, between their flat sides, they have an air gap. At one of the two short sides of the rectangular configuration, those plates are connected by a limb portion which extends perpendicularly to the planes of the plates, in such a way as to provide a U-shaped longitudinal section. The limb portion extends through an exciter coil which is fed with alternating current to generate a magnetic flux which can follow an annularly closed path, across the air gap, with a substantially homogeneous magnetic field being produced in the air gap.
That flux guide means is connected to one of the two mutually movable bodies while coupled to the other body is a measurement coil arrangement which is in the form of a printed circuit and which has two measurement coils of which each includes a plurality of windings each embracing a respective surface element.
The surface elements which are formed in that way are of different sizes and arranged in interleaved relationship with each other. All in all, that arrangement is of an elongated measurement coil configuration which extends in the direction of the movement to be monitored, and defines the maximum width thereof. The carrierboard of the measurement coils is disposed between the two plates of the flux guide means and parallel to said plates. The two mutually oppositely disposed wall surfaces of the air gap formed between the plates of the flux guide means concentrate the magnetic flux which crosses over between them, on to a substantially parallelepipedic spatial region which intersects the surface of the measurement coil carrierboard and thus the measurement coil at a substantially rectangular transit surface, the longitudinal direction of which extends perpendicularly to the direction of movement transversely across the entire measurement coil arrangement, and the width of which, in the direction of movement, is substantially shorter than the maximum width of movement. If one of the two bodies to be monitored moves relative to the other body, then the transit surface is displaced over the surface elements of the measurement coil arrangement, whereby the magnetic flux passing through the individual windings changes so that the measurement coils respectively produce an electrical output signal of variable amplitude. The output ac voltage signals produced by the measurement coils are rectified so as to give variable dc voltage signals, the respective magnitude of which is characteristic of the instantaneous position occupied by one of the two bodies with respect to the other. In order to suppress interference influences and to produce an output signal which is symmetrical relative to the zero potential associated with the central position, the two measurement coils are of such a configuration and/so arranged in mirror image relationship with each other that regions of the transit surface which, upon movement of the two bodies, leave the condition of overlap with the one measurement coil, pass into a condition of overlap with the other measurement coil and vice-versa; in that situation, the difference signal from the output signals of the two measurement coils is to follow a linear configuration as accurately as possible, over as large a part as possible of the range of the movement to be monitored. In the case of configurations in which another output signal characteristic is predetermined, it is desirable in a corresponding fashion for the characteristic which is theoretically predetermined by the selected configuration to be maintained as accurately as possible over the entire range of movement.
That requirement is only met to an inadequate degree, by the state of the art. Thus for example it can be readily seen from German patent specification No. 25 11 683 that the linear configuration of the output signal characteristic, which that arrangement seeks to achieve, terminates not in a sharp point but in a rounded point, when approaching the two limit positions. It is also found in practice that the characteristic of such an arrangement also extends not linearly but distorted in a S-shaped form, in the region of the passage through zero. Admittedly, such non-linearities can be partially compensated by means of the electronic circuitry disposed on the output side of the indicator. However that involves additional structure and thus increased costs.